1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a data transmission/reception apparatus and method for achieving both multiplexing gain and diversity gain in a mobile communication system using a space-time trellis code (hereinafter referred to as “STTC”).
2. Description of the Related Art
With the rapid development of mobile communication systems, the amount of data serviced by the mobile communication system has also increased. Recently, a 3rd generation mobile communication system for transmitting high-speed data has been developed. For the 3rd generation mobile communication system, Europe has adopted an asynchronous wideband-code division multiple access (hereinafter referred to as “W-CDMA”) system as its radio access standard, while North America has adopted a synchronous code division multiple access-2000 (hereinafter referred to as “CDMA-2000”) system as its radio access standard. Generally, in these mobile communication systems, a plurality mobile stations (MSs) communicate with each other via a common base station (BS). However, during high-speed data transmission in the mobile communication system, a phase of a received signal may be distorted due to a fading phenomenon occurring on a radio channel. The fading reduces amplitude of a received signal by several dB to several tens of dB. If a phase of a received signal distorted due to the fading phenomenon is not compensated for during data demodulation, the phase distortion becomes a cause of information error of transmission data transmitted by a transmission side, causing a reduction in the quality of a mobile communication service. Therefore, in mobile communication systems, fading must be overcome in order to transmit high-speed data without a decrease in the service quality, and several diversity techniques are used in order to cope with the fading.
Generally, a CDMA system adopts a rake receiver that performs diversity reception by using delay spread of a channel. While the rake receiver applies reception diversity for receiving a multipath signal, a rake receiver applying the diversity technique using the delay spread is disadvantageous in that it does not operate when the delay spread is less than a preset value. In addition, a time diversity technique using interleaving and coding is used in a Doppler spread channel. However, the time diversity technique is disadvantageous in that it can hardly be used in a low-speed Doppler spread channel.
Therefore, in order to cope with fading, a space diversity technique is used in a channel with low delay spread, such as an indoor channel, and a channel with low-speed Doppler spread, such as a pedestrian channel. The space diversity technique uses two or more transmission/reception antennas. In this technique, when a signal transmitted via one transmission antenna decreases in its signal power due to fading, a signal transmitted via the other transmission antenna is received. The space diversity can be classified into a reception antenna diversity technique using a reception antenna and a transmission diversity technique using a transmission antenna. However, since the reception antenna diversity technique is applied to a mobile station, it is difficult to install a plurality of antennas in the mobile station in view of the mobile station's size and its installation cost. Therefore, it is recommended that the transmission diversity technique should be used in which a plurality of transmission antennas are installed in a base station.
Particularly, in a 4th generation mobile communication system, a data rate of about 10 Mbps to 150 Mbps is expected, and an error rate requires a bit error rate (hereinafter referred to as “BER”) of 10−3 for voice, BER of 10−6 for data, and BER of 10−9 for image. The STTC is a combination of a multi-antenna technique and a channel coding technique, and is a technique bringing a drastic improvement of a data rate and reliability in a radio MIMO (Multi Input Multi Output) channel. The STTC obtains the receiver's space-time diversity gain by extending the space-time dimension of a transmitter's transmission signal. In addition, the STTC can obtain coding gain without a supplemental bandwidth, contributing to an improvement in channel capacity.
Therefore, in the transmission diversity technique, the STTC is used. When the STTC is used, coding gain having an effect of increasing transmission power is obtained together with diversity gain which is equivalent to a reduction in channel gain occurring due to a fading channel when the multiple transmission antennas are used. A method for transmitting a signal using the STTC is disclosed in Vahid Tarokh, N. Seshadri, and A. Calderbank, “Space Time Codes For High Data Rate Wireless Communication: Performance Criterion And Code Construction,” IEEE Trans. on Info. Theory, pp. 744-765, Vol. 44, No. 2, March 1998. In this reference, it is provided that if a code rate is defined as the number of symbols transmitted for a unit time, the code rate must be smaller than 1 in order to obtain diversity gain corresponding to the product of the number of transmission antennas and the number of reception antennas.
FIG. 1 is a block diagram schematically illustrating a general structure of a transmitter using STTC. Referring to FIG. 1, when L information data bits d1, d2, d3, . . . , dL are input to the transmitter, the input information data bits d1, d2, d3, . . . , dL are provided to a serial-to-parallel (S/P) converter 111. Here, the index L represents the number of information data bits to be transmitted by the transmitter for a unit transmission time, and the unit transmission time can become a symbol unit. The S/P converter 111 parallel-converts the information data bits d1, d2, d3, . . . , dL and provides its outputs to first to Lth encoders 121-1 to 121-L. That is, the S/P converter 111 provides a parallel-converted information data bit d1, to the first encoder 121-1, and in this manner, provides a parallel-converted information data bit dL to the Lth encoder 121-L. The first to Lth encoders 121-1 to 121-L each encode signals output from the S/P converter 111 in a predetermined encoding scheme, and then each provide their outputs to first to Mth modulators 131-1 to 131-M. Here, the index M represents the number of transmission antennas included in the transmitter, and the predetermined encoding scheme is an STTC encoding scheme. A detailed structure of the first to Lth encoders 121-1 to 121-L will be described later with reference to FIG. 2.
The first to Mth modulators 131-1 to 131-M each modulate signals received from the first to Lth encoders 121-1 to 121-L in a predetermined modulation scheme. The first to Mth modulators 131-1 to 131-M are similar to one another in operation except the signals applied thereto. Therefore, only the first modulator 131-1 will be described herein. The first modulator 131-1 adds up signals received from the first to Lth encoders 121-1 to 121-L, multiplies the addition result by a gain applied to a transmission antenna to which the first modulator 131-1 is connected, i.e., a first transmission antenna ANT#1, modulates the multiplication result in a predetermined modulation scheme, and provides the modulation result to the first transmission antenna ANT#1. Here, the modulation scheme includes BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation), PAM (Pulse Amplitude Modulation), and PSK (Phase Shift Keying). It will be assumed in FIG. 1 that since the number of encoders is L, 2L-ary QAM is used as a modulation scheme. The first to Mth modulators 131-1 to 131-M provide their modulation symbols S1 to SM to first to Mth transmission antennas ANT#1 to ANT#M, respectively. The first to Mth transmission antennas ANT#1 to ANT#M transmit to the air the modulation symbols S1 to SM output from the first to Mth modulators 131-1 to 131-M.
FIG. 2 is a block diagram illustrating a detailed structure of the first to Lth encoders 121-1 to 121-L of FIG. 1. For simplicity, a description will be made of only the first encoder 121-1. The information data bit d1 output from the S/P converter 111 is applied to the first encoder 121-1, and the first encoder 121-1 provides the information data bit d1 to tapped delay lines, i.e., delays (D) 211-1, 211-2, . . . , 211-(K−1). Here, the number of the delays, or the tapped delay lines, is smaller by 1 than a constraint length K of the first encoder 121-1. The delays 211-1, 211-2, . . . , 211-(K−1) each delay their input signals. That is, the delay 211-1 delays the information data bit d1 and provides its output to the delay 211-2, and the delay 211-2 delays an output signal of the delay 211-1. In addition, input signals provided to the delays 211-1, 211-2, . . . , 211-(K−1) are multiplied by predetermined gains, and then provided to modulo adders 221-1, . . . , 221-M, respectively. The number of the modulo adders is identical to the number of the transmission antennas. In FIG. 1, since the number of the transmission antennas is M, the number of the modulo adders is also M. Further, gains multiplied by the input signals of the delays 211-1, 211-2, . . . , 211-(K−1) are represented by gi,j,t, where i denotes an encoder index, j an antenna index and t a memory index. In FIG. 1, since the number of encoders is L and the number of antennas is M, the encoder index i increases from 1 to L, the antenna index j increases from 1 to M, and the memory index K increases from 1 to the constraint length K. The modulo adders 221-1, . . . , 221-M each modulo-add signals obtained by multiplying the input signals of the corresponding delays 211-1, 211-2, . . . , 211-(K−1) by the gains. The STTC encoding scheme is also disclosed in Vahid Tarokh, N. Seshadri, and A. Calderbank, “Space Time Codes For High Data Rate Wireless Communication: Performance Criterion And Code Construction,” IEEE Trans. on Info. Theory, pp. 744-765, Vol. 44, No. 2, March 1998.
FIG. 3 is a block diagram schematically illustrating a structure of an STTC transmitter having two encoders and 3 transmission antennas. Referring to FIG. 3, when 2 information data bits d1 and d2 are input to the transmitter, the input information data bits d1, and d2 are applied to an S/P converter 311. The S/P converter 311 parallel-converts the information data bits d1 and d2, and outputs the information data bit d1 to a first encoder 321-1 and the information data bit d2 to a second encoder 321-2. If it is assumed that the first encoder 321-1 has a constraint length K of 4 (constraint length K=4), an internal structure, illustrated in FIG. 2, of the first encoder 321-1 is comprised of 3 delays (1+2D+D3) and 3 modulo adders, wherein the number of delays and modulo adders is equal to a value smaller by 1 than the constant length K=4. Therefore, in the first encoder 321-1, the undelayed information data bit d1 applied to a first delay, a bit determined by multiplying a bit delayed once by the first delay by 2, and a bit delayed three times by a third delay are provided to a first modulo adder connected to a first modulator 331-1 of a first transmission antenna ANT#1. In this manner, outputs of the 3 modulo adders of the first encoder 321-1 are provided to the first modulator 331-1, a second modulator 331-2 and a third modulator 331-3, respectively. Similarly, the second encoder 321-2 encodes the information data bit d2 output from the S/P converter 311 in the same encoding scheme as that used by the first encoder 321-1, and then, provides its outputs to the first modulator 331-1, the second modulator 331-2 and the third modulator 331-3.
The first modulator 331-1 modulates the signals output from the first encoder 321-1 and the second encoder 321-2 in a predetermined modulation scheme, and then provides its output to a first transmission antenna ANT#1. It is assumed herein that a modulation scheme applied to the transmitter is QPSK. Therefore, if an output signal of the first encoder 321-1 is b1 and an output signal of the second encoder 321-2 is b2, the first modulator 331-1 modulates the output signals in the QPSK modulation scheme, and outputs b1+b2*j, where j=√{square root over (−1)}. Like the first modulator 331-1, the second modulator 331-2 and the third modulator 331-3 modulate output signals of the first encoder 321-1 and the second encoder 321-2 in the QPSK modulation scheme, and then, provide their outputs to a second transmission antenna ANT#2 and a third transmission antenna ANT#3, respectively. The first to third transmission antennas ANT#1 to ANT#3 transmit to the air the modulation symbols S1 to S3 output from the first to third modulators 331-1 to 331-3, respectively.
FIG. 4 is a block diagram schematically illustrating a receiver structure corresponding to the transmitter structure using the STTC described above in conjunction with FIG. 1. Referring to FIG. 4, a signal transmitted to the air by a transmitter is received through reception antennas of the receiver. It is assumed in FIG. 4 that there are provided N reception antennas. The N reception antennas each process signals received from the air. Specifically, a signal received through a first reception antenna ANT#1 is provided to a channel estimator 411 and a metric calculator 423. The channel estimator 411 performs channel estimation on signals output from the first to Nth reception antennas ANT#1 to ANT#N, and then provides the channel estimation result to a hypothesis part 412.
A possible sequence generator 415 generates all kinds of sequences which were possibly simultaneously encoded for information data bits transmitted by the transmitter, and provides the generated sequences to first to Lth encoders 417-1 to 417-L. Since the transmitter transmits information data by the L information bits, the possible sequence generator 415 generates possible sequences {tilde over (d)}1 . . . {tilde over (d)}L comprised of L bits. The L bits of the generated possible sequences are applied to the first to Lth encoders 417-1 to 417-L, and the first to Lth encoders 417-1 to 417-L encode their input bits in the STTC encoding scheme described in conjunction with FIG. 2, and then provide the encoded bits to first to Mth modulators 419-1 to 419-M. The first to Mth modulators 419-1 to 419-M each modulate the encoded bits output from the first to Lth encoders 417-1 to 417-L in a predetermined modulation scheme, and provide their outputs to the hypothesis part 412. The modulation scheme applied in the first to Mth modulators 419-1 to 419-M is set to any one of the BPSK, QPSK, QAM, PAM and PSK modulation schemes. Since a modulation scheme applied in the first to Mth modulators 131-1 to 131-M of FIG. 1 is 2L-ary QAM, the first to Mth modulators 419-1 to 419-M also modulate their input signals in the 2L-ary QAM modulation scheme.
The hypothesis part 412 receives signals output from the first to Mth modulators 419-1 to 419-M and the channel estimation value output from the channel estimator 411, generates a hypothetic channel output at a time when a sequence consisting of the signals output from the first to Mth modulators 419-1 to 419-M has passed a channel corresponding to the channel estimation result, and provides the generated hypothetic channel output to the metric calculator 423. The metric calculator 423 receives the hypothetic channel output provided from the hypothesis part 412 and the signals received through the first to Nth reception antennas ANT#1 to ANT#N, and calculates a distance between the hypothetic channel output and the signals received through the first to Nth reception antennas ANT#1 to ANT#N. The metric calculator 423 uses Euclidean distance when calculating the distance.
In this manner, the metric calculator 423 calculates Euclidean distance for all possible sequences the transmitter can transmit, and then provides the calculated Euclidean distance to a minimum distance selector 425. The minimum distance selector 425 selects a Euclidean distance having the minimum distance from Euclidean distances output from the metric calculator 423, determines information bits corresponding to the selected Euclidean distance as information bits transmitted by the transmitter, and provides the determined information bits to a parallel-to-serial (P/S) converter 427. Although there are several possible algorithms used when the minimum distance selector 425 determines information bits corresponding to the Euclidean distance having the minimum distance, it is assumed herein that a Viterbi algorithm is used. A process of extracting information bits having the minimum distance by using the Viterbi algorithm is disclosed in Vahid Tarokh, N. Seshadri, and A. Calderbank, “Space Time Codes For High Data Rate Wireless Communication: Performance Criterion And Code Construction,” IEEE Trans. on Info. Theory, pp. 744-765, Vol. 44, No. 2, March 1998, so a detailed description thereof will not be provided for simplicity. Since the minimum distance selector 425 determines information bits corresponding to the Euclidean distance having the minimum distance for all sequences generated from the possible sequence generator 415, it finally outputs L information bits of {circumflex over (d)}1, {circumflex over (d)}1, . . . , {circumflex over (d)}L. The P/S converter 427 then serial-converts the L information bits output from the minimum distance selector 425, and outputs reception information data sequences {circumflex over (d)}1, {circumflex over (d)}1, . . . , {circumflex over (d)}L.
As described above, when the transmitter transmits a signal with a plurality of transmission antennas, the STTC can achieve coding gain having an effect of amplifying power of a received transmission signal, together with diversity gain, in order to prevent a reduction in channel gain occurring due to a fading channel. In Tarokh, it is provided that if a code rate is defined as the number of symbols transmitted for a unit time in a communication system using STTC, the code rate must be smaller than 1 in order to obtain diversity gain corresponding to the product of the number of transmission antennas and the number of reception antennas. That is, it is provided that if it is assumed that the number of information data bits in a symbol transmitted to the air through one transmission antenna at a particular transmission time is N, even though a transmitter uses a plurality of transmission antennas, the number of information data bits that can be transmitted to the air through the plural transmission antennas at a particular transmission time must be smaller than or equal to N in order to achieve diversity gain corresponding to the product of the number of transmission antennas and the number of reception antennas. The reason for providing that the number of information data bits that can be transmitted to the air through a plurality of transmission antennas should be smaller than or equal to N is to maintain diversity gain through the plural transmission antennas.
As mentioned above, a mobile communication system using STTC can achieve both the diversity gain and the coding gain, so the system is effective when using multiple antennas in a varying channel environment. However, since only one data stream is transmitted through multiple antennas, it is difficult to achieve multiplexing gain, which is equivalent to achieving gain in terms of a data rate. In order to solve this problem, there has been recently proposed a technique for applying multiplexing to multiple antennas before transmission in a transmitter in order to maximize a multiplexing gain, i.e., a data rate. In a technique for applying channel coding to the multiple antennas, a transmitter transmits a plurality of data streams through plural transmission antennas, thereby achieving both diversity gain and multiplexing gain.
Meanwhile, in a technique for applying STTC to the multiple antennas, if the number of transmission antennas of a transmitter is 3 and the number of reception antennas of a receiver is 3, it is possible in theory to obtain 9-level diversity gain. However, in practice in an actual mobile communication system, diversity gain of over 4 levels does not affect improvement in system performance, so there is a limitation on improvement in system performance. In a technique proposed to make up for the defects that result in system performance which cannot be improved further even though high-level diversity gain can be actually obtained, when the number of transmission antennas of a transmitter is larger than or equal to a predetermined number, the transmission antennas are classified into several groups for signal transmission. The technique for classifying the transmission antennas into several groups for signal transmission is called “combined array processing and diversity.” The combined array processing and diversity technique is disclosed in Vahid Tarokh, A. Naguib, N. Seshadri, and A. Calderbank, “Combined Array Processing And Space Time Coding.” IEEE Trans. Inform. Theory, Vol. 45, pp. 1121-1128, May 1999.
FIG. 5 is a block diagram schematically illustrating a general structure of an STTC transmitter using the combined array processing and diversity technique. Referring to FIG. 5, the transmitter includes M transmission antennas, and classifies the M transmission antennas into P groups. That is, MP transmission antennas constitute one group, and each group performs the transmission operation, i.e., encoding and modulation operations, described in conjunction with FIG. 1. Here, the sum of M1 to MP is M. The combined array processing and diversity technique will now be described with reference to a first transmission antenna group and a Pth transmission antenna group among the P transmission antenna groups.
First, the first transmission antenna group will be described. If L information data bits d11, d21, d31, . . . , dL1 are input to a transmitter of the first transmission antenna group, the input information data bits d11, d21, d31, . . . , dL1 are provided to an S/P converter 511. Here, the index L represents the number of information data bits to be transmitted by the transmitter of the first transmission antenna group for a unit transmission time, and the unit transmission time can become a symbol unit. In addition, the index “1” succeeding the index L represents the first transmission antenna group. The S/P converter 511 parallel-converts the information data bits d11, d21, d31, . . . , dL1 and provides its outputs to first to L1th encoders 521-1 to 521-L1. That is, the S/P converter 511 provides a parallel-converted information data bit d11 to the first encoder 521-1, and in this manner, provides a parallel-converted information data bit dL1 to the L1th encoder 521-L1. The first to L1th encoders 521-1 to 521-L1 each encode signals output from the S/P converter 511 in a predetermined encoding scheme, and then provide their outputs to first to M1th modulators 531-1 to 531-M1. Here, the index M1 represents the number of transmission antennas included in the transmitter of the first transmission antenna group, and the predetermined encoding scheme is an STTC encoding scheme.
The first to M1th modulators 531-1 to 531-M1 each modulate signals received from the first to L1th encoders 521-1 to 521-L1 in a predetermined modulation scheme. The first to M1th modulators 531-1 to 531-M1 provide modulation symbols S1 to SM1 to first to M1th transmission antennas ANT#1 to ANT#M1, respectively. The first to M1th transmission antennas ANT#1 to ANT#M1 transmit to the air the modulation symbols S1 to SM1 output from the first to M1th modulators 531-1 to 531-M1.
Second, the Pth transmission antenna group will be described. If L information data bits d1P, d2P, d3P, . . . , dLP are input to a transmitter of the Pth transmission antenna group, the input information data bits d1P, d2P, d3P, . . . , dLP are provided to an S/P converter 551. Here, the index “P” succeeding the index L represents the Pth transmission antenna group. The S/P converter 551 parallel-converts the information data bits d1P, d2P, d3P, . . . , dLP and provides its outputs to first to LPth encoders 561-1 to 561-LP. That is, the S/P converter 551 provides a parallel-converted information data bit d1P to the first encoder 561-1, and in this manner, provides a parallel-converted information data bit dLP to the LPth encoder 561-LP. The first to LPth encoders 561-1 to 561-LP each encode signals output from the S/P converter 551 in a predetermined encoding scheme, and then provide their outputs to first to MPth modulators 571-1 to 571-MP. Here, the index MP represents the number of transmission antennas included in the transmitter of the Pth transmission antenna group.
The first to MPth modulators 571-1 to 571-MP each modulate signals received from the first to LPth encoders 561-1 to 561-LP in a predetermined modulation scheme. The first to MPth modulators 571-1 to 571-MP provide modulation symbols S1 to SMP to (M−MP+1)th to Mth transmission antennas ANT#(M−MP+1) to ANT#M, respectively. The (M−MP+1)th to Mth transmission antennas ANT#(M−MP+1) to ANT#M transmit to the air the modulation symbols S1 to SMP output from the first to MPth modulators 571-1 to 571-MP.
As described in conjunction with FIG. 5, the combined array processing and diversity classifies M transmission antennas into P transmission antenna groups, and then modulates input information data according to the groups before transmission, thereby increasing diversity gain efficiency. In addition, the combined array processing and diversity technique transmits a non-overlapping signal through transmission antennas.
FIG. 6 is a block diagram schematically illustrating a receiver structure based on the combined array processing and diversity technique, and corresponding to the transmitter structure of FIG. 5. Referring to FIG. 6, a signal transmitted to the air by a transmitter is received through reception antennas of the receiver. It is assumed in FIG. 6 that there are provided N reception antennas. The N reception antennas each process signals received from the air. Specifically, signals received through first to Nth reception antennas ANT#1 to ANT#N are provided to a channel estimator 611 and an interference suppressor 613. The channel estimator 611 performs channel estimation on signals output from the first to Nth reception antennas ANT#1 to ANT#N, and then provides the channel estimation result to the interference suppressor 613. The interference suppressor 613 removes an interference component from each of the signals output from the first to Nth reception antennas ANT#1 to ANT#N based on the channel estimation result output from the channel estimator 611, and then provides its outputs to first to Pth decoders 615-1 to 615-P. Considering signals output from the first to Nth reception antennas ANT#1 to ANT#N, of the N reception antennas, ΣMPp={2˜p} reception antennas are used to remove the interference component and the other reception antennas are used to increase diversity gain. A process of removing by the interference suppressor 613 an interference component from the signals received from the first to Nth reception antennas ANT#1 to ANT#N is also disclosed in Vahid Tarokh, A. Naguib, N. Seshadri, and A. Calderbank, “Combined Array Processing And Space Time Coding.” IEEE Trans. Inform. Theory, Vol. 45, pp. 1121-1128, May 1999, so a detailed description thereof will be omitted for simplicity. The interference component-removed signals output from the interference compressor 613 are provided to the first to Pth decoders 615-1 to 615-P. The first to Pth decoders 615-1 to 615-P each perform STTC decoding on signals output from the interference compressor 613, and output {circumflex over (d)}11{circumflex over (d)}21{circumflex over (d)}31 . . . {circumflex over (d)}L1 to {circumflex over (d)}1P{circumflex over (d)}2P{circumflex over (d)}3P . . . {circumflex over (d)}LP.
The combined array processing and diversity technique can simply trade off a diversity gain, i.e., a diversity order, with a data rate. In order to increase the diversity order, the number of transmission antenna groups of a transmitter must be increased. In addition, a receiver can relatively simply remove an interference component through the operation of removing the interference component. However, the combined array processing and diversity brings about a great loss in diversity gain in the process of trading off the diversity gain with the data rate. For example, it will be assumed that a transmitter has 3 transmission antennas and a receiver also has 3 reception antennas. The transmitter forms two transmission antennas into a first transmission antenna group, and forms the remaining one transmission antenna into a second transmission antenna group. Thus, it will be assumed that the transmitter transmits a first stream through the first transmission antenna group and a second stream through the second transmission antenna group. In this case, the receiver removes the second stream that acts as an interference component when decoding the first stream, thereby obtaining a diversity gain of a level 4. However, the receiver removes the first stream that acts as an interference component when decoding the second stream, so it has a diversity gain of a level 1, and this operates as if there is no diversity gain. Therefore, the combined array processing and diversity technique has a great loss of diversity gain when the number of transmission antennas of the transmitter is small.
In order to eliminate the diversity gain loss of the combined array processing and diversity technique, there has been proposed a technique for transmitting a signal by overlapping a plurality of transmission antennas, and the technique for transmitting a signal by overlapping the transmission antennas is called “overlapped combined array processing and diversity.” The overlapped combined array processing and diversity technique is disclosed in Korean patent application No. 2002-59621, filed on Sep. 30, 2002, and commonly assigned to the assignee of this application, the contents of which are incorporated herein by reference. This reference discloses a technique for transmitting/receiving a signal by grouping transmission antennas so that some transmission antennas among the transmission antennas overlap one another.
FIG. 7 is a block diagram schematically illustrating a general structure of an STTC transmitter based on the overlapped combined array processing and diversity technique. Referring to FIG. 7, the transmitter includes M transmission antennas, and classifies the M transmission antennas into P groups. That is, MP transmission antennas constitute one group, and each group performs the transmission operation, i.e., encoding and modulation operations, described in conjunction with FIG. 1. Here, the sum of M1 to MP exceeds M. The reason that the sum of M1 to MP exceeds M is because the overlapped combined array processing and diversity technique fundamentally overlaps transmission antennas. The overlapped combined array processing and diversity technique will now be described with reference to a first transmission antenna group and a Pth transmission antenna group among the P transmission antenna groups.
First, the first transmission antenna group will be described. If L information data bits d11, d21, d31, . . . , dL1 are input to a transmitter of the first transmission antenna group, the input information data bits d11, d21, d31, . . . , dL1 are provided to an S/P converter 711. Here, the index L represents the number of information data bits to be transmitted by the transmitter of the first transmission antenna group for a unit transmission time, and the unit transmission time can become a symbol unit. In addition, the index “1” succeeding the index L represents the first transmission antenna group. The S/P converter 711 parallel-converts the information data bits d11, d21, d31, . . . , dL1 and provides its outputs to first to L1th encoders 721-1 to 721-L1. That is, the S/P converter 711 provides a parallel-converted information data bit d11, to the first encoder 721-1, and in this manner, provides a parallel-converted information data bit dL1 to the L1th encoder 721-L1. The first to L1th encoders 721-1 to 721-L1 each encode signals output from the S/P converter 711 in a predetermined encoding scheme, and then provide their outputs to first to M1th modulators 731-1 to 731-M1. Here, the index M1 represents the number of transmission antennas included in the transmitter of the first transmission antenna group, and the predetermined encoding scheme is an STTC encoding scheme.
The first to M1th modulators 731-1 to 731-M1 each modulate signals received from the first to L1th encoders 721-1 to 721-L1 in a predetermined modulation scheme. The first to M1th modulators 731-1 to 731-M1 provide modulation symbols S1 to SM1−1 to a first summer 741-1. Here, the summers are matched to the transmission antennas on a one-to-one basis, and the first summer 741-1 is connected to a first transmission antenna ANT#1. Of the modulation symbols S1 to SM1, the modulation symbol SM1 is provided even to the second summer 741-2, and the reason is because a signal output from the M1th modulator 731-M1 among output signals of the first transmission antenna group overlaps with output signals of a second transmission antenna group. The summer 741-1 sums up the modulation symbols S1 to SM1 and transmits the summation result to the air through the first transmission antenna ANT#1.
Second, the Pth transmission antenna group will be described. If L information data bits d1P, d2P, d3P, . . . , dLP are input to a transmitter of the Pth transmission antenna group, the input information data bits d1P, d2P, d3P, . . . , dLP are provided to an S/P converter 751. Here, the index “P” succeeding the index L represents the Pth transmission antenna group. The S/P converter 751 parallel-converts the information data bits d1P, d2P, d3P, . . . , dLP and provides its outputs to first to LPth encoders 761-1 to 761-LP. That is, the S/P converter 751 provides a parallel-converted information data bit dip to the first encoder 761-1, and in this manner, provides a parallel-converted information data bit dLP to the LPth encoder 761-LP. The first to LPth encoders 761-1 to 761-LP each encode signals output from the S/P converter 751 in an STTC encoding scheme, and then provide their outputs to first to MPth modulators 771-1 to 771-MP. Here, the index MP represents the number of transmission antennas included in the transmitter of the Pth transmission antenna group.
The first to MPth modulators 771-1 to 771-MP each modulate signals received from the first to LPth encoders 761-1 to 761-LP in a predetermined modulation scheme. The first to MPth modulators 771-1 to 771-MP provide modulation symbols S1 to SMP to an Mth summer 741-M. Of the modulation symbols S1 to SMP, the modulation symbol S1 is provided even to the second summer 741-2, and the reason is because a signal output from the first modulator 771-1 among output signals of the Pth transmission antenna group overlaps with output signals of the second transmission antenna group. The summer 741-M sums up the modulation symbols S1 to SM1 and transmits the summation result to the air through the Mth transmission antenna ANT#M.
FIG. 8 is a block diagram schematically illustrating a receiver structure based on the overlapped combined array processing and diversity technique, and corresponding to the transmitter structure of FIG. 7. Referring to FIG. 8, a signal transmitted to the air by a transmitter is received through reception antennas of the receiver. It is assumed in FIG. 8 that there are provided N reception antennas. The N reception antennas each of which process signals received from the air. Specifically, signals received through first to Nth reception antennas ANT#1 to ANT#N are provided to a channel estimator 811 and an interference suppressor 813. The channel estimator 811 performs channel estimation on signals output from the first to Nth reception antennas ANT#1 to ANT#N, and then provides the channel estimation result to the interference suppressor 813. The interference suppressor 813 removes an interference component from each of the signals output from the first to Nth reception antennas ANT#1 to ANT#N based on the channel estimation result output from the channel estimator 811, and then provides its outputs to first to Pth decoders 815-1 to 815-P. Considering signals output from the first to Nth reception antennas ANT#1 to ANT#N, of the N reception antennas, (M−MP) reception antennas are used to remove the interference component and the other reception antennas are used to increase diversity gain. A process of removing by the interference suppressor 813 an interference component from the signals received from the first to Nth reception antennas ANT#1 to ANT#N is disclosed in Korean patent application No. 2002-59621, filed on Sep. 30, 2002, and commonly assigned to the assignee of this application, and is hereby incorporated by reference. A detailed description thereof will be omitted for simplicity. The interference component-removed signals output from the interference compressor 813 are provided to the first to Pth decoders 815-1 to 815-P. The first to Pth decoders 815-1 to 815-P each perform STTC decoding on signals output from the interference compressor 813, and output {circumflex over (d)}11{circumflex over (d)}21{circumflex over (d)}31 . . . {circumflex over (d)}L1 to {circumflex over (d)}1P{circumflex over (d)}2P{circumflex over (d)}3P . . . {circumflex over (d)}LP. In the receiver based on the overlapped combined array processing and diversity technique illustrated in FIG. 8, a diversity gain becomes N−M+MP.
The overlapped combined array processing and diversity technique, as mentioned above, uses an overlapping method when grouping transmission antennas, so it can have a higher diversity gain as compared with the combined array processing and diversity technique. However, due to the overlapping method, even though the receiver eliminates an interference component, the interference component may exist, so that parallel transition is permitted in a trellis diagram. For example, when a transmitter has 3 transmission antennas and a receiver also has 3 reception antennas, a first stream is transmitted through a first transmission antenna and a second stream is transmitted through a second transmission antenna. In this case, information on the first transmission stream is added to information on the second transmission stream, and transmitted through the second transmission antenna. The receiver then performs interference suppression on a signal transmitted through a third transmission antenna only for a signal of the second stream when decoding the first stream, so the receiver has a diversity gain of a level 4 by achieving diversity gain for the 2 reception antennas. Likewise, the receiver is permitted to perform interference suppression on a signal transmitted from the first transmission antenna corresponding to only a signal of the first stream when decoding the second stream, so the receiver has a diversity gain of a level 4 by achieving diversity gain for the 2 reception antennas. However, as to a signal transmitted from the second transmission antenna according to the overlapping method, its modulation order is increased undesirably, since the first stream and the second stream overlap each other during transmission. For example, if modulation symbols of a transmission stream are 16QAM symbols, a signal transmitted from the second transmission antenna becomes a 256QAM signal obtained by overlapping 16QAM symbols. The 256QAM symbol and the 16QAM symbol are different from each other in their peak-to-average power ratio (hereinafter referred to as “PAPR”), and disadvantageously require design modification for a radio frequency (RF) processor. Finally, the overlapped combined array processing and diversity technique is disadvantageous in that it must consider parallel transition as mentioned above. A trellis structure that considers the parallel transition will be described with reference to FIG. 13.
FIG. 13 illustrates a trellis structure based on the overlapped combined array processing and diversity technique. Since the overlapped combined array processing and diversity technique has the trellis structure that considers the parallel transition as illustrated in FIG. 13, an error rate may be increased undesirably due to the parallel transition, and in addition, an amount of trellis calculation is doubled undesirably due to the parallel transition.